The recent demand for energy saving has been tending to increase the number of highly airtight, highly insulated houses, contributing to the increase in demand for air conditioning duct unit for use in such highly airtight, highly insulated houses. Also, hotels and office buildings have conventionally been provided with a central air conditioning system where each room is air-conditioned with air that is conditioned by an air conditioner and supplied through an air conditioning duct extending from the air conditioner to each room. In such an air conditioning system using an air conditioning duct unit or an air conditioning duct, an air-conditioning-duct outlet port is formed in a wall or the ceiling of a room, and conditioned air is introduced into the room through the outlet port.
In one example of such an air conditioning system which has been proposed, there is provided, inside an air conditioning duct, an ion generating device for generating negative ions which are said to have a relaxing effect on human mind and body, such that negative ions generated by the ion generating device are supplied to a room (see Patent Literature 1).